Such types of externally-heated, regenerative heat engines possess good prospects for wider utilization in the future because of their low emission of exhaust gases, their very high attainable efficiency and their adaptability to the most diverse sources of energy supply. An example of this type of heat engine is the Stirling engine.
Although heat engines of this type operate according to an inherently ideal, reversible cycle, which evolves between two isotherms and two isochores, and therefore have a fundamentally higher Carnot cycle efficiency than do the much more widely-used internal combustion engines, up till the present time they have not been able to establish themselves on a widespread basis for several different reasons.
A problem resides in the face that, in order to achieve a high degree of efficiency, the temperature of the heater head must be very high and it is only in the most recent times that suitable materials have been developed for this purpose. A further fundamental problem arises from the fact that, for achieving a good power density associated with an appropriately small compact engine, the engine must be filled with helium or hydrogen under high pressure. This leads, for example in the known crankshaft Stirling engines, to very considerable difficulties because, on the one hand, the sealing of the crankshaft is supposed to avoid the loss by leakage of the working gas and, on the other hand, to prevent the ingress of oil and lubricants from the gearbox into the interior of the engine. An improvement for the solution of this problem was presented by the invention of the free-piston Stirling engine (U.S. Pat. No. Re. 30,176).
This type of arrangement does not require a crankshaft because not only the working piston, but also the displacement piston, supported freely and resiliently by gas or mechanical means, undergoes linear oscillatory motion within an hermetically-sealed pressure housing. This gives rise to a mass-spring resonance system which is, or should be, tuned by selection of the magnitudes of the spring constants, mass- and flow-cross-sections, in such a manner that as close an approach as possible is made to the movement kinematics of the ideal Stirling cycle.
With the use of such a free-piston engine, in contrast to the construction employing a crankshaft, the advantage is obtained that it is completely gas-tight and therefore no diffusion of gas can take place from the inside to the outside of the engine and there can be no penetration of oil from the outside into the engine. In addition to this, the construction, at least as far as the elementary components are concerned, is uncomplicated and makes a relatively long working life a distinct possibility. The end result is that an increased degree of efficiency may be attained, because there are no losses due to the crankshaft operation.
However, there are problems involved in the practical utilization of such a type of engine. Because all the mechanical movements take place only in the interior of the engine and there is no mechanical connection with the outside whatsoever, it is necessary to employ additional auxiliary devices, even within the engine itself, for the transformation of the oscillatory movement of the working piston into useful thermal or electrical energy. This leads to the situation where, for example, in contrast to the use of conventional, commercially-available, rotating generators for the production of electrical current, use must be made of relatively-expensive linear generators.
Even greater problems arise if it is desired to derive mechanical energy as such from the engine. To this end, experiments have been conducted in an attempt to transfer the periodic pressure fluctuations within the engine to an hydraulic system by way of a membrane. Another attempt at a solution consists of constructing the working piston with a great enough weight so that the energy of its linear oscillation can be exploited by way of the related movement of the housing which results from the engine being mounted on a flexible bearing. Both of these attempts at solution of the problem lead, in actual practice, to very considerable technical complications. There is, for example, the problem of what type of material to use for the membrane, and also the difficulties which arise in the transformation of the linear oscillatory motion of the engine housing into a rotary movement.
An additional fundamental problem with free-piston machines is seen in the fact that it is extraordinarily difficult to deal analytically with such a mass-spring resonance system. In this regard, it is especially the question of centering the piston around the middle position, as well as the phase angle between the displacement- and working-piston, which causes the difficulty. For conversion from the theoretical considerations, measures such as the provision of by-pass channels for the gas, intermediate storage for the gas, spring for mechanical centering, and the like, were adopted. All of these measures reduce the ideal efficiency and only operate over severly-limited ranges, thus leading to deviations from the ideal Stirling cycle.